The present invention relates to an antenna device for portable communication equipment, specifically for hand portable telephones, comprising circuits for transmitting and/or receiving radio signals as well as a chassis and a feeding point providing the electrical coupling of the antenna device to the communication equipment, the antenna device comprising a hollow helical antenna fixed externally on the chassis and an antenna rod slidable through the helical antenna, which antenna rod includes a substantially straight radiator.
Antennas for hand-portable telephones may be divided generally into two main groups, quarter-wave and half-wave radiators, although types somewhere between and outside these may occur.
Characteristics of a quarter-wave radiator are:
the length of its actively radiating portion is one quarter of a wavelength (radiating portion length approximately 8 cm at 900 MHz), PA1 its feeding connection impedance is low, which allows its direct connection to the 50 Ohm impedance of the telephone, without impedance transformation, PA1 due to its short length a 900 MHz telephone user will not consider it disturbing, PA1 it is dependent on a ground plane for its function. PA1 the length of its actively radiating portion is one half of a wavelength (radiating portion length approximately 16 cm at 900 MHz), PA1 its feeding connection impedance is high, which requires impedance transformation to the 50 Ohm impedance of the telephone, PA1 it is unsuitable for use in a small telephone due to its total length of 18-20 cm including a connector, PA1 it is independent of a ground plane for its function.
Characteristics of a half-wave length radiator are:
In the specification and claims below the terms half-wave radiator and quarter-wave radiator refer to antennas having substantially the above characteristics, respectively.
A straight radiator mounted on the outside of the chassis of a hand portable telephone occupies considerable space and is exposed to strain which, for example can result in breaking. Further, only lower antenna performance is required when no call is going on. For this reason, the straight radiator is often made retractable and is combined with a helical antenna having lower antenna performance, but being considerably less bulky.
U.S. Pat. No. 4,868,576 describes an antenna device of the aforementioned type, the device comprising a retractable capacitively coupled half-wave radiator in combination with a helical antenna of half-wave type. This known device solves the above problem for a telephone of large size, but is dependent on a half-wave radiator to provide satisfactory antenna performance. However, the physical dimensions of modern hand portable telephones are so small that a half-wave radiator would be very difficult to fit therein lengthwise, without providing with a complicated telescopically retracting feature. It is also lacks an aesthetically pleasing appearance since its length is out of proportion to the small size of modern telephones.
Further, the half-wave radiator requires an impedance transformer taking up room inside the telephone and causing higher costs due to additional components.
The European Patent Application 0 467822 A2 discloses another antenna system for a hand portable telephone comprising a retractable quarter-wave radiator provided in its upper end with a helical antenna of the quarter-wave type, which substantially has no coupling to the quarter-wave radiator. The quarterwave radiator is only connected in the fully extended position and the helical antenna only in the fully retracted position. This systems solves the above-mentioned problem for a small-size telephone, but has the drawback of the telephone completely lacking antenna function as soon as the retractable quarterwave radiator is not in one of its end positions. Besides, it is both an aesthetic and a strength drawback to provide the helical antenna in the upper end of the quarter-wave radiator.